ATLAS Accelerator Research Articles

Triaxiality and the nature of low-energy excitations in Ge76

The deformation properties of the low-lying states in Ge76 have been investigated following a safe-energy Coulomb excitation measurement with the GRETINA tracking array and CHICO2 heavy-ion counter at the ATLAS accelerator facility at Argonne National Laboratory. A comprehensive set of transition and static E2 matrix elements were extracted from the measured differential Coulomb cross sections and compared with results of configuration-interaction shell-model calculations and computations carried out within the framework of the generalized triaxial rotor model. The remarkable agreement between the calculated and experimental data supports a near-maximum triaxial deformation for the ground state of Ge76. In addition, the degree of softness of the asymmetry in Ge76 and Se76 was investigated using rotational invariants generated from configuration-interaction shell-model wave functions computed with the jj44b and JUN45 effective interactions. The resulting invariants are shown to be consistent with a stiff triaxial deformation in Ge76 and a predominantly soft triaxial potential for Se76, in agreement with the conclusions of recent works by this collaboration.5 MoreReceived 27 January 2023Accepted 3 April 2023DOI:https://doi.org/10.1103/PhysRevC.107.044314©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNuclear reactionsNuclear structure & decaysNucleon distributionProperties59 ≤ A ≤ 89TechniquesCollective modelsShell modelNuclear Physics

Rhenium and iridium targets prepared using a novel graphene loading technique

For accelerator targets, graphene films are an excellent material choice due to their high thermal conductivity, high temperature tolerance, low outgassing, mechanical integrity, and ease of handling. A variety of targets have been produced using graphene material as a backing or a host matrix. One of the unique advantages of the graphene film fabrication process is the capability to embed target materials, including refractory metals, in the nanoparticle form into a host graphene matrix during target preparation. Targets of natIr and natRe have been fabricated as nanoparticle loaded graphene targets for use in nuclear physics research. We have obtained beam time to evaluate target performance as well as production yields and nuclear decay properties via the natRe(a,2n)186Ir and natIr(a,3n)194Au reactions, respectively. These rhenium and iridium targets will be irradiated using the ATLAS accelerator and gamma rays measured in-place using the high-precision gamma-ray spectroscopy capabilities of Gammasphere and further analyzed using a multi-parameter detector system. Future plans include the preparation of isotopic targets of these two elements.

Charge breeding of radioactive isotopes at the CARIBU facility with an electron beam ion source.

An Electron Beam Ion Source Charge Breeder (EBIS-CB) has been developed at Argonne National Laboratory as part of the californium rare ion breeder upgrade. For the past year, the EBIS-CB has been undergoing commissioning as part of the ATLAS accelerator complex. It has delivered both stable and radioactive beams with A/Q < 6, breeding times <30 ms, low background contamination, and charge breeding efficiencies >18% into a single charge state. The operation of this device, challenges during the commissioning phase, and future improvements will be discussed.

The ATLAS multi-user upgrade and potential applications

With the recent integration of the CARIBU-EBIS charge breeder into the ATLAS accelerator system to provide for more pure and efficient charge breeding of radioactive beams, a multi-user upgrade of the ATLAS facility is being proposed to serve multiple users simultaneously. ATLAS was the first superconducting ion linac in the world and is the US DOE low-energy Nuclear Physics National User Facility. The proposed upgrade will take advantage of the continuous-wave nature of ATLAS and the pulsed nature of the EBIS charge breeder in order to simultaneously accelerate two beams with very close mass-to-charge ratios; one stable from the existing ECR ion source and one radioactive from the newly commissioned EBIS charge breeder. In addition to enhancing the nuclear physics program, beam extraction at different points along the linac will open up the opportunity for other potential applications; for instance, material irradiation studies at ∼1 MeV/u, isotope production and radiobiological studies at ∼6 MeV/u and at the full ATLAS energy of ∼15 MeV/u. The concept and proposed implementation of the ATLAS multi-user upgrade will be discussed. Future plans to enhance the flexibility of this upgrade will be presented.

Investigation of the mixed-symmetric one-quadrupole phonon 2+1,ms state of the heavy nucleus 204Hg

Low-energy quadrupole-collective excitations of isovector character, the so-called states with proton-neutron mixed-symmetric character (MSS), have been investigated in 204Hg which is the sole stable nucleus in the straight vicinity of 208Pb exhibiting a 2π — 2ν structure. A 12C(204Hg,204Hg*)12C projectile Coulomb-excitation experiment at 890MeV was performed at Argonne National Laboratory using the ATLAS accelerator. γ-rays originating from the decay of the excited states of 204Hg were detected using the GAMMASPHERE spectrometer. The measured Coulomb-excitation yields provide the B(M1) and the B(E2) strength distributions which unambiguously reveal the 23+ state at 1948 keV as the one-phonon MSS of 204Hg.

Structure ofC14andB14from theC14,15(d,He3)B13,14reactions

We have studied the $^{14,15}\mathrm{C}(d,^{3}\mathrm{He})^{13,14}\mathrm{B}$ proton-removing reactions in inverse kinematics. The $(d,^{3}\mathrm{He})$ reaction probes the proton occupation of the target ground state, and also provides spectroscopic information about the final states in $^{13,14}\mathrm{B}$. The experiments were performed using $^{14,15}\mathrm{C}$ beams from the ATLAS accelerator at Argonne National Laboratory. The reaction products were analyzed with the HELIOS device. Angular distributions were obtained for transitions from both reactions. The $^{14}\mathrm{C}$-beam data reveal transitions to excited states in $^{13}\mathrm{B}$ that suggest configurations with protons outside the $\ensuremath{\pi}(0{p}_{3/2})$ orbital, and some possibility of proton cross-shell $0p\text{\ensuremath{-}}1s0d$ excitations, in the $^{14}\mathrm{C}$ ground state. The $^{15}\mathrm{C}$-beam data confirm the existence of a broad ${2}^{\ensuremath{-}}$ excited state in $^{14}\mathrm{B}$. The experimental data are compared to the results of shell-model calculations.

Reactions on the surface and inside of neutron stars

Measurements from orbiting X-ray satellites during the last decades have provided us with a wealth of information about nuclear reactions thought to occur in the extreme, highdensity environment of neutron stars. With radioactive ion beams from first-generation facilities we have begun to study some of these processes in the laboratory. In this contribution I report on experiments performed with radioactive beams from the ATLAS accelerator at Argonne. I will discuss the nuclear physics of X-ray bursts and super-bursts, the production of in-flight radioactive beams, as well as novel detectors which are used in these experiments.

New Information on the Occurrence of the O(6) Symmetry in Nuclei

New γγ -coincidence relations and E 2 decay transition rates in the isotopes 194,196 Pt have been deduced from γ -ray spectroscopy experiments using the Gammasphere spectrometer in projectile-Coulomb excitation reactions of beams of 194,196 Pt ions provided by the ATLAS accelerator facility. The results give access to observables that are crucial for a classification of excited quadrupole-collective states in terms of quantum numbers associated with the analytically solvable O(6) dynamical symmetry of the interacting boson model. The data on 196 Pt corroborate the qualitative pattern of excitation energies and E 2 transition rates expected from the O(6) solution but the excitation energies significantly deviate from it on a quantitative level.

Astrophysics experiments with radioactive beams at ATLAS

Reactions involving short-lived nuclei play an important role in nuclear astrophysics, especially in explosive scenarios which occur in novae, supernovae or X-ray bursts. This article describes the nuclear astrophysics program with radioactive ion beams at the ATLAS accelerator at Argonne National Laboratory. The CARIBU facility as well as recent improvements for the in-flight technique are discussed. New detectors which are important for studies of the rapid proton or the rapid neutron-capture processes are described. At the end we briefly mention plans for future upgrades to enhance the intensity, purity and the range of in-flight and CARIBU beams.

Study of207Tl126Produced in Deep-Inelastic Reactions

Deep-inelastic collisions of a 208 Pb beam on a 208 Pb target were performed us- ing the ATLAS accelerator at Argonne National Laboratory. The Gammasphere detector array was used for the detection of prompt and delayed gamma-rays of the reaction prod- ucts. 207 Tl is one proton away from the 208 Pb doubly-magic nucleus. Its low-energy level structure is dominated by the single proton-hole states s 1

Preparation of isotopic antimony targets

Thin self-supporting 123Sb targets were needed for studies using GAMMASPHERE investigating transverse wobbling in the highly-deformed triaxial nucleus 135Pr. The experiment was carried out using the 123Sb(16O,4n)135Pr reaction with the 80 MeV 16O beam provided by the ATLAS accelerator facility. In particle–particle coincidence measurements 121Sb targets were irradiated with a 332 MeV 28Si beam from ATLAS to measure evaporation residues and fission. The antimony targets were prepared self-supporting by the method of physical vapor deposition onto polished glass substrates or on various backing materials. Target thicknesses on the order of 500–1,000 μg/cm2 were obtained and used for the experiments. Details of the target production and performance in beam will be discussed.

Relative quadrupole moments of exotic shapes at ultrahigh spin in 154Er: calibrating the TSD/SD puzzle

Transition quadrupole moments, Qt, of two ultrahigh-spin, collective structures in 154Er have been measured for the first time using the Doppler Shift Attenuation Method (DSAM). Data were acquired at the ATLAS accelerator facility of Argonne National Laboratory, using the Gammasphere detector array. A thick, gold-backed 110Pd foil was bombarded by a beam of 48Ti ions at 215 MeV. The Qt for each band was determined from the Doppler shift of gamma rays emitted by the resulting recoil nuclei. The extracted transition quadrupole moments are significantly different in magnitude, suggesting the two structures in 154Er represent distinct exotic nuclear shapes, namely axial superdeformed (SD) with Qt ≈ 20 eb, and triaxial strongly deformed (TSD) with Qt ≈ 11 eb. Indeed, the results calibrate the quadrupole moments of TSD bands recently measured in light erbium nuclei, 157,158Er.

NEW STRUCTURES IN 178HF AND COULOMB EXCITATION OF ISOMERS

A 985 MeV 178 Hf beam was Coulomb excited by a 208 Pb target at the ATLAS accelerator of Argonne National Laboratory. Gammasphere and the CHICO particle detector recorded particle-γ coincidence data. The aim was to populate and determine the mechanism of previously observed Coulomb excitation of the Kπ = 6+ (t1/2 = 77 ns ), 8- (4 s ) and 16+ (31 y ) isomer bands. New rotational bands were identified including an aligned band which appears to mix with the ground-state band (GSB) and the γ-vibrational band above ~ 12 ħ of angular momentum. Newly observed γ-decay transitions into the three isomer bands may elucidate the K-mixing which allows Coulomb excitation of these isomer bands, but direct decays from the GSB into the 16+ isomer band have not yet been confirmed.

Simple Atom, Extreme Nucleus-Laser Trapping and Probing of 6He and 8He

Helium-8 (8He) is the most neutron-rich helium isotope that can be synthesized on Earth: it consists of two protons and six neutrons, and remains stable for an average of 0.2 seconds. Unlike the abundant helium isotope, 4He, which has the two neutrons packed closely with two protons, the additional neutrons in 8He form a “halo” around a compact, 4He-like core. A similar neutron-halo structure also exists for 6He (half-life = 0.8 s). Because of their intriguing properties, 6He and 8He have the potential to reveal new aspects of the fundamental forces among the constituent nucleons. We have recently succeeded in laser trapping and cooling these exotic helium isotopes, and have performed precision laser spectroscopy on individual trapped atoms. Based on atomic frequency differences measured along the isotope chain 4He-6He-8He, the nuclear charge radius of 8He has been determined for the first time to be 1.96 fm and that of 6He to be 2.07 fm. The result shows that the charge radius of 8He is smaller than that of 6He, but notably larger than that of 4He (1.68 fm). This may be understood by noting that the four extra neutrons are more evenly distributed around the 4He core than the extra neutron pair in 6He as illustrated in Figure 1. The result is in good agreement with recent predictions of state-of-the-art nuclear structure theories, and provides a critical test of the present understanding of these loosely bound halo nuclei. The 6He measurement was first performed at the ATLAS accelerator facility of Argonne National Laboratory in 2004. An improved measurement on 6He and the first measurement on 8He were performed at GANIL in 2007.

The effect of annealing at 1500 °C on migration and release of ion implanted silver in CVD silicon carbide

The transport of silver in CVD β-SiC has been studied using ion implantation. Silver ions were implanted in β-SiC using the ATLAS accelerator facility at the Argonne National Laboratory. Ion beams with energies of 93 and 161 MeV were used to achieve deposition with peak concentrations of approximately 26 wt% at depths of approximately 9 and 13 μm, respectively. As-implanted samples were then annealed at 1500 °C for 210 or 480 h. XPS, SEM, TEM, STEM, and optical methods were used to analyze the material before and after annealing. Silver concentration profiles were determined using XPS before and after annealing. STEM and SEM equipped with quantitative chemical analysis capability were used to more fully characterize the location and morphology of the silver before and after annealing. The results show that, within the uncertainty of measurement techniques, there is no silver migration, via either inter- or intra-granular paths, for the times and temperature studied. Additionally, the silver was observed to phase separate within the SiC after annealing. The results of this work do not support the long held assumption that silver release from CVD SiC, used for gas-reactor coated particle fuel, is dominated by grain boundary diffusion.

Extreme-ultraviolet wavelength and lifetime measurements in highly ionized krypton

We have studied the spectrum of highly ionized krypton in the extreme-ultraviolet wavelength region (50–300 Å), using beam-foil excitation of fast krypton ions at the Argonne ATLAS accelerator facility. We report measurements of transition wavelengths and excited-state lifetimes for n = 2 states in the lithiumlike, berylliumlike, and boronlike ions, Kr31+,32+,33+. Excited state lifetimes ranging from 10 ps to 3 ns were measured by acquiring time-of-flight-delayed spectra with a position-sensitive multichannel detector.PACS Nos.: 32.70.Cs, 32.30.Jc, 34.50.Fa

First studies of the 8B( α,p) 11C reaction

The 8B(α,p)11C reaction is part of the network that can bypass the triple α process leading to the production of 12C. We have measured the astrophysical reaction rate for this reaction by studying the inverse 11C(p,α)8B process. The radioactive 11C beam was produced via the p(11B,11C)n reaction using the in-flight facility at the ATLAS accelerator. The astrophysical reaction rate obtained from the excitation function measured in the energy range Ex=8.8-10 MeV was found to be a factor of 10-50 higher than previous estimates.

Superconducting linac beam dynamics with high-order maps for RF resonators

The arbitrary-order map beam optics code COSY Infinity has recently been adapted to calculate accurate high-order ion-optical maps for electrostatic and radio-frequency accelerating structures. The beam dynamics of the superconducting low-velocity positive-ion injector linac for the ATLAS accelerator at Argonne National Lab is used to demonstrate some advantages of the new simulation capability. The injector linac involves four different types of superconducting accelerating structures and has a total of 18 resonators. The detailed geometry for each of the accelerating cavities is included, allowing an accurate representation of the on- and off-axis electric fields. The fields are obtained within the code from a Poisson-solver for cylindrically symmetric electrodes of arbitrary geometry. The transverse focusing is done with superconducting solenoids. A detailed comparison of the transverse and longitudinal phase space is made with the conventional ray-tracing code LINRAY. The two codes are evaluated for ease of optimization, with particular attention to higher-order effects, and future applications are discussed.

Measurement of the 8B neutrino spectrum

The neutrino spectrum from the decay of 8B is a crucial ingredient in interpreting recent data from solar neutrino detectors. The β + decay of 8B proceeds to a broad state in 8Be, and the shape of the neutrino spectrum may be obtained from a measurement of the α spectrum following the β + decay. A new technique has been used at the ATLAS accelerator to measure this spectrum by implanting 8B particles into the mid-plane of a 91 μm thick Si detector. The advantage of this method is that both α particles are detected and systematic effects due to energy loss in catcher foils and dead layers of the detector are eliminated. To calibrate the detector, α's from the decay of 20Na ions produced and implanted with the same technique were used.

Study of the 8B neutrino spectrum with a new technique

The neutrino spectrum from the decay of 8B is a crucial ingredient in interpreting recent data from solar neutrino detectors. Since 8B decays to a broad state in 8Be, the shape of the neutrino spectrum is best obtained from a measurement of the α spectrum following the β + decay. A new technique has been used at the ATLAS accelerator to measure this spectrum by implanting 8B particles into the mid-plane of a 91 μm thick Si detector. The advantage of this method is that both α particles are detected and systematic effects due to energy loss in catcher foils and dead layers of the detector are eliminated. To calibrate the detector, α's from the decay of 20Na ions produced and implanted with the same technique were used.